Method for the rectifying separation of fluids containing (meth)acryl monomers

ABSTRACT

In a process for rectificatively separating (meth)acrylic monomer-containing fluids, a rectification column is employed which has at least one sieve tray without runoff segment in which the passages are arranged regularly.

The present invention relates to a process for rectificatively.separating (meth)acrylic monomer-containing fluids (=the fluid fed tothe rectification column) in a rectification column which comprises atleast one sieve tray without runoff segment.

The notation (meth)acrylic monomers in this document is an abbreviationof “acrylic monomers and/or methacrylic monomers”.

The term acrylic monomers in this document is an abbreviation of“acrolein, acrylic acid and/or esters of acrylic acid”.

The term methacrylic monomer in this document is an abbreviation of“methacrolein, methacrylic acid and/or esters of methacrylic acid”.

In particular, the (meth)acrylic monomers referred to in this documentinclude the following (meth)acrylic esters: hydroxyethyl acrylate,hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropylmethacrylate, glycidyl acrylate, glycidyl methacrylate, methyl acrylate,methyl methacrylate, n-butyl acrylate, isobutyl acrylate, isobutylmethacrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butylmethacrylate, ethyl acrylate, ethyl methacrylate, 2-ethylhexyl acrylate,2-ethylhexyl methacrylate, N,N-dimethylaminoethyl acrylate andN,N-dimethylaminoethyl methacrylate.

(Meth)acrylic monomers are important starting compounds for preparingpolymers which find use, for example, as adhesives.

(Meth)acroleinand (meth)acrylic acid are prepared on the industrialscale predominantly by catalytic gas phase oxidation of suitable C₃/C₄precursor compounds, in particular of propene and propane in the case ofacrolein and acrylic acid, and of isobutene and isobutane in the case ofmethacrylic acid and methacrolein. However, in addition to propene,propane, isobutene and isobutane, other useful starting materials areother compounds containing 3 or 4 carbon atoms, for example isobutanol,n-propanol or the methyl ether as the C₄ precursor of isobutanol.(Meth)acrylic acid may also be obtained from (meth)acrolein.

A product gas mixture is normally obtained from which the (meth)acrylicacid or the (meth)acrolein has to be removed.

This removal is generally carried out in such a manner that the(meth)acrylic acid or (meth)acrolein undergoes initial basic removal byabsorption into a solvent (for example water or an organic solvent) orby fractional condensation of the product gas mixture and the resultingcondensate or absorbate is subsequently separated rectificatively (ingeneral in more than one stage) to obtain more or less pure(meth)acrylic acid or (meth)acrolein (cf., for example, EP-A 717019,EP-A 1125912, EP-A 982289, EP-A 982287, DE-A 19606877, DE-A 1011527,DE-A 10224341, DE-A 10218419). In this document, the abovementionedfractional condensation shall be regarded as falling under thedefinition of rectification. It differs from conventional rectificationonly in that the mixture to be separated is fed to the separating column(the rectification column) in gaseous form (i.e. completely converted tovapor form). The term fluids used in this document shall thereforeencompass both liquids and gas mixtures.

Esters of (meth)acrylic acid are obtainable, for example, by directreaction of (meth)acrylic acid and/or (meth)acrolein with theappropriate alcohols. However, product mixtures also occur in this casefrom which the (meth)acrylic ester has to be removed, for examplerectificatively.

The abovementioned (meth)acrylic monomer-containing fluids or liquidsmay comprise the (meth)acrylic monomers either in more or less pure formor in solution.

The solvent may either be aqueous or an organic solvent. The specifictype of the solvent is substantially insignificant to the invention. Thecontent of (meth)acrylic monomers may be ≧2% by weight, ≧5% by weight,or ≧10% by weight, or ≧20% by weight, or ≧40% by weight, or ≧60% byweight, or ≧80% by weight, or ≧90% by weight, or ≧95% by weight, or ≧99%by weight.

Depending on their composition, the fluids or liquids comprising the(meth)acrylic monomers described can be rectificatively separated eitherin such a manner that the (meth)acrylic monomers accumulate at the topof the rectification column or in such a manner that the (meth)acrylicmonomers accumulate in the bottom of the rectification column. It willbe appreciated that the (meth)acrylic monomer-enriched fractions may bealso be withdrawn in the upper, lower or middle section of therectification column.

In all cases (in particular the abovementioned), rectification columnswhich comprise at least one sieve tray without runoff segment (cf. forexample, DE-A 19924532) may be used for the rectificative separation inquestion. However, it will be appreciated that these rectificationcolumns may also comprise exclusively sieve trays without runoffsegments as the sole separating internals of the rectification column(cf., for example, DE-A 10156988 and EP-A 1029573). It is also possibleto use other separating internals, for example bubble cap trays orstructured packings.

The rectification may be carried out either under atmospheric pressureor under reduced pressure. Typical bottom temperatures are in the rangefrom 100 to 250° C. and typical top pressures are from 80 to 500 mbar.

The solvents frequently accompanying the (meth)acrylic monomers oftencomprise diphenyl, for example mixtures of diphenyl ether, diphenyl ando-dimethyl phthalate. An example of a solvent which is frequently usedfor the absorption of (meth)acrylic monomers comprises about 57.4% byweight of diphenyl ether, 20.7% by weight of diphenyl and 20% by weightof o-dimethyl phthalate. Other solvents frequently accompanying(meth)acrylic acid and (meth)acrylic esters are methyl acrylate andethyl acrylate.

In general, the rectification column in the rectifications discussed inthis document is polymerization-inhibited using polymerizationinhibitors. These are customarily introduced in the top of the column,but may in addition also be added to the liquid phase and also alreadyhave been added to the liquid to be separated which comprises the(meth)acrylic monomers. Typical representatives of such polymerizationinhibitors include phenothiazine, 2-methoxyphenol and4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl. Based on the content of(meth)acrylic monomers, up to a few hundred ppm by weight ofpolymerization inhibitors are frequently used.

Now that the (meth)acrylic monomer-containing fluids and liquids whichare particularly relevant to this document have been described in detailabove, the sieve trays without runoff segments shall now be consideredin detail. In the literature, the term dual-flow trays is alsofrequently used for such trays. In this document, this term isconsidered to refer to plates having simple passages (holes, slits,etc.) which in many cases are also referred to as trickle sieve trays.

The absence of runoff segments (runoff shafts) causes the rising gas andthe reflux liquid falling in the column to flow in opposite directionsthrough the same passages of the tray. The cross section of the passagesis adjusted in a manner known per se to the loading of the column. Whereit is too small, the rising gas flows through the passages at such highspeed that the reflux liquid falling in the column is entrainedsubstantially without separation. When the cross section of the passagesis too large, rising gas and falling reflux move past each othersubstantially without exchange and the tray is at the risk of runningdry. In other words, the working range for dual flow trays is defined by2 limiting points. There has to be a minimum limiting speed, so that acertain liquid layer is maintained on the tray in order to allow thetray to work. The upper limit is defined by the flood point when thespeed leads to liquid build-up on the tray and prevents trickle through.In the normal working range, the liquid refluxing in the rectificationcolumn trickles in drops from dual-flow tray to dual-flow tray, i.e.between the trays, the continuous gas phase is interspersed by a dividedliquid phase. Some of the drops striking the trickle sieve tray areatomized.

In general, each dual-flow tray is connected flush to the column walls.However, there are also embodiments in which there is an intermediatespace between column wall and tray which is only partially interruptedby bridges. In addition to the actual passages, it may, if need be, havefurther orifices which facilitate, for example, securing of the tray tosupport rings or the like (cf., for example, DE-A 10159823).

For rectificative treatment of (meth)acrylic monomer-containing liquids,the earlier application DE-A 10156988 recommends rectification columnshaving dual-flow trays having passages whose cross section, althoughconstant within one dual-flow tray, decreases with increasing separationof the tray from the feed of the liquid to be treated.

According to the teaching of DE-A 10156988, the longest extent of thepassages is typically from 10 to 80 mm. However, a disadvantage of theteaching of DE-A 10156988 is that it makes no statements as to how thepassages within one dual-flow tray should be arranged relative to oneanother. The same applies to the teaching of EP-A 1029573. In bothdocuments, recommendation is merely made of circular drillholes aspreferred passages.

However, intensive in-house investigations have shown that the relativearrangement of the passage of a dual-flow tray has a decisive influenceon the separating performance of the tray in the rectificativeseparation of (meth)acrylic monomer-containing fluids or liquids. Thisis true in particular when the (meth)acrylate monomer content of thefluid or of the liquid is from 2 to 5% by weight, or from 10 to 35% byweight, or ≧95% by weight.

In this context, it was also found that the irregular triangular pitch(the location of the centers of the drillholes deviates distinctly froma location on theoretical straight lines) graphically represented inFIG. 1 of EP-A 1029573 as a possible relative arrangement is notcompletely satisfactory.

It is an object of the present invention to provide an improved relativearrangement of the passages of the trickle sieve tray for therectificative separation of (meth)acrylic monomer-containing fluids (inparticular the abovementioned fluids) in a rectification column whichhas at least one sieve tray without runoff segment to the effect thatthe trickle sieve tray has an improved separating performance.

We have found that this object is achieved by a process forrectificatively separating (meth)acrylic monomer-containing fluids in arectification column which comprises at least one sieve tray without arunoff segment,

wherein

both the arrangement of the centers of the passages present in the sievetray and the arrangement of the passages themselves are regular and canbe obtained from a basic number of these centers and passages bydefining a rectangular elementary cell in the sieve tray which containsthe basic number and has four edges, of which two in each case areparallel to one another and of equal length, and this elementary cellshifts regularly and repeatedly along its edges, the length of theshifting vector in each case being the length of the edge of theelementary cell along which the shifting is effected.

According to the invention, two edges of the elementary cell areregarded as parallel to one another when the angle formed by theirdirections is ≦0.2°. The abovementioned angle is preferably less than 10angle minutes, more preferably less than 5 angle minutes and mostpreferably less than 2 angle minutes. At best, this angle is 0°.

According to the invention, two parallel edges of the elementary cellare in a similar manner regarded as being equally long when thedifference in length, based on half of the sum of their lengths, is ≦1%,preferably ≦0.5%, more preferably ≦0.25%, and most preferably ≦0.1%. Atbest, the lengths are identical. When the lengths are not equal, thelength of the shifting vector according to the claim is equal to half ofthe sum of the lengths. Normally, the elementary cell will not containmore than ten centers of passages. Usually, it contains ≦8, frequently≦5 or ≦4, centers of passages.

Normally, this number of centers of an elementary cell is ≦10%, usually≦7%, in many cases ≦5% and frequently ≦3% of all the centers of passagescontained in the trickle sieve tray.

The center of a passage refers to the theoretical center of mass whichresults when the passage is filled with a homogeneous mass which has thesame filling thickness at each point in the orifice.

In principle, the passages of the at least one trickle sieve tray to beused according to the invention may have any geometric shape. In otherwords, the passages may be circles, ellipses, rectangles, triangles,polygons or slits.

According to the invention, all passages of an at least one tricklesieve tray to be used according to the invention advantageously have thesame geometric shape and the same cross section. In this case, thedifference between the largest and the smallest cross section of thepassages captured by the elementary cell is preferably ≦1%, better≦0.75%, better still ≦0.5%, better still ≦0.25%, better still ≦0.l% andat best 0%, based on the surface area of the largest cross sectioncaptured. This geometric shape is preferably circular. However, it willbe appreciated that there may also be passages of different geometricshape and different cross section within the elementary cells relevantto the invention.

The regular arrangement of the centers required according to theinvention should be regarded as fulfilled when the regularly repeatingshifting of the elementary cell along its edges results, in at least 90%(preferably at least 95%, more preferably at least 98%, even morepreferably at least 99% or 99.9% and at best 100%) of all cases, in theposition of the ideal image of the center generated by the shifting andthe position of the corresponding real center present in the sieve traybeing separated by ≦1%, preferably ≦0.75%, more preferably by ≦0.5%,even more preferably by ≦0.3% and even better by ≦0.1%, of half of thesum of the lengths of the two possible shifting vectors. In general,this separation between ideal and real center in the appropriate numberof cases will be ≦0.1 mm, preferably ≦0.05 mm and more preferably ≦0.02mm. Ideally, ideal and real centers coincide in all cases.

The regular arrangement of the passages required according to theinvention should be regarded as fulfilled when, on the one hand, theabovementioned regular arrangement of the centers is fulfilled in eachcase (i.e. the abovementioned condition in each case is fulfilled) and,on the other hand, the regularly repeating shifting of the elementarycell along its edges results in at least 90% (preferably at least 95%,more preferably at least 98%, even more preferably at least 99% or 99.9%and at best 100%) of all cases in the surface area of the ideal passagegenerated by the shifting and the surface area of the associated realpassage present in the sieve tray overlapping with each other to such anextent that the sum of the nonoverlapping residual surface area of theideal passage generated by the shifting and the nonoverlapping residualsurface area of the corresponding real passage, based on the surfacearea of the ideal passage generated by the shifting, (referred tohereinbelow as S_(Rem.)) is ≦1%, preferably ≦0.75%, more preferably≦0.5%, even more preferably ≦0.3% and even better ≦0.1%. Ideally,S_(Rem.)=0.

Elementary cells advantageous according to the invention have thegeometry of a rectangle (cf. FIG. 1, 3 (rectangular face-centeredelementary cell) and 5), of a square (cf. FIG. 2) or of a rhombus (cf.FIG. 4). This is also the case when the relationship d=a·{square root}3in FIG. 3 between the longer edge d and the shorter edge a is notfulfilled or the edges in FIG. 4 are not at equal length and/or theangle between them is not 60°. Particular preference is given to anarrangement of the passages in which the elementary cell is aface-centered square (i.e. corresponding to FIG. 3 with four identicaledge lengths).

Otherwise, the at least one trickle sieve tray without runoff segment tobe used may be configured and arranged in the column as described inDE-A 10156988 or in EP-A 1029573.

Processes according to the invention are therefore those in whichrectification columns are used which, as separating internals, havetrays of whose number at least one, preferably more than one (preferably≧10%, or ≧20%, or ≧30%, or ≧40%, or ≧50%, or ≧60%, or ≧75%, of alltrays) and more preferably all are trickle sieve trays according to theinvention, of which trickle sieve trays having circular passages arepreferred. It is advantageous when, in addition, all circular orificeswithin one trickle sieve tray have the same cross section.

Preference is given according to the invention to processes inrectification columns whose separating internals are exclusively tricklesieve trays of the type to be used according to the invention. This istrue in particular when the passages are circular, and in particularwhen the hole diameter is varied from tray to tray according to DE-A10156988. However, they may also be constant over all trays. Thethickness of the at least one trickle sieve tray to be used in theprocess according to the invention is advantageously from 2 mm to 12 mm.

According to the invention, the orifice ratio (ratio of the totalsurface area of all passages of the at least one trickle sieve tray tobe used according to the invention to the total surface area of thistrickle sieve tray) is generally advantageously from 0.1 to 0.3.

The separation T between two neighboring centers in an at least onetrickle sieve tray to be used according to the invention is customarily1.2 d to 3 d, where d is the length of the longest extent of the largerpassage (in the case of a circle the circle diameter). d is typicallyfrom 10 to 80 mm, frequently from 10 to 25 mm.

When the rectification column used in the process according to theinvention has two or more successive trickle sieve trays to be usedaccording to the invention, the separation is advantageously from 0.1 Dto 0.5 D, where D is the diameter of the trays or the internal diameterof the rectification column.

The trickle sieve trays to be used according to the invention arepreferably manufactured from stainless steel, in particular stainlesssteel 1.4571 (according to DIN EN 10020).

Finally, it is once again emphasized that the result of the presentinvention is that trickle sieve trays having strictly regulararrangements of the passages are superior to trickle sieve trays havingirregular arrangement of the passages with regard to the rectificativeseparation performance of (meth)acrylic monomer-containing fluids orliquids. It will be appreciated that the rectification columns to beused according to the invention may also be used for other thermalseparating processes, for example extractions or strippings, or forrectificative separations of other fluids or liquids.

Trickle sieve trays to be used according to the invention are notablefor low tendency to polymer formation.

It is self-evident that it is possible in the peripheral area of the atleast one sieve tray without runoff segment only to apply part of theelementary cell to the passages present there.

The process according to the invention is suitable in particular for theprocesses of fractional condensation or rectification described in DE-A19924532, DE-A 10115277, EP-A 982289, EP-A 982287 and EP-A 982288.

EXAMPLES (THE TRAY NUMBERING RISES WITHIN THE COLUMN FROM THE BOTTOM TOTHE TOP) Example 1

120 t per hour of a mixture of 67% by weight of Diphyl® (mixture ofabout 25% by weight of diphenyl and about 75% by weight of diphenylether), 16% by weight of dimethyl phthalate, 15.8% by weight of acrylicacid, 300 ppm by weight of phenothiazine and the remainder of smallamounts of compounds such as benzaldehyde, acetic acid, propionic acid,furfurals, formic acid and formaldehyde was fed to a tray column whichhad 40 equidistant (40 cm) dual-flow trays. The rectification column wasoperated at a reflux ratio of 2.2. The column bottom was heated with aforced-circulation evaporator and the vapors were condensed with aninjection condenser which injected reflux cooled to a temperature of 45°C. The bottom temperature was 214° C., the top pressure 225 mbar. Themixture was fed to the 10th tray. At tray 32, the removed acrylic acidwas withdrawn. For experimental purposes, trays 30, 31 and 32 wereexchangeable; below tray 30 30 (at tray 29) and immediately above tray32, a sample withdrawal device was installed which allowed a sample tobe withdrawn from the appropriate liquid phase. Above the 40th tray, aliquid distributor was installed which distributed about 60 t per hourof reflux, stabilized with 280 ppm by weight of phenothiazine, into therectification column having a diameter of 3.50 m. The distributorconsisted of three cubic boxes arranged in parallel and open at the topwhich have a length of 3 m, a width of 20 cm and a height of 25 cm. Theupper end of each box was configured as a toothed weir.

After a running time of 10 days, samples were taken at theabovementioned sampling points. The withdrawn acrylic acids had thecompositions listed in Table 1.

The relative arrangement of the passages in trickle sieve trays 30 to 32was as follows:

-   -   elementary cell: ideally as in FIG. 3 where d=a·{square root}3        and d is the longer and a the shorter edge of the elementary        cell; d=58.88 mm.    -   the elementary cell contained five centers of passages, four of        them on the corners of the elementary cell;    -   the passages were all circular and had a uniform target diameter        of 15 mm which was fulfilled in all cases within a range of        variation of ±0.01 mm;    -   the separation between two neighboring centers within the        elementary cell was 34 mm;    -   the orifice ratio was 0.16;    -   the regularly repeating shifting of the elementary cell along        its edges resulted in 99.6% of all cases in the position of the        image of the ideal center generated by the shifting and the        position of the corresponding real center present in the sieve        tray being separated from one another by 50.1% of the length of        half of the shifting vector sum;

regularly repeating shifting of the elementary cell along its edgesresulted in 99.8% of all cases in S_(Rem.)≦0.1%. TABLE 1 Below tray 30Above tray 32 Acrylic acid 99.12% by weight 99.58% by weight

Comparative Example

As in Example 1, except that trays 30, 31 and 32 were replaced by traysaccording to FIG. 1 of EP-A 1029573. FIG. 6 of this document which is anoriginal copy of the abovementioned FIG. 1 assumes the deviations fromthe ideal line.

In all three exchanged trays, the prerequisites of the trays fromExample 1 were fulfilled with from the following two exceptions:

-   -   regularly repeating shifting of the elementary cell along its        edges resulted only in 89% of all cases in the position of the        image of the ideal center generated by the shifting and the        position of the corresponding real center present in the sieve        tray being separated by ≦1% of the length of half of the        shifting vector. sum;    -   regularly repeating shifting of the elementary cell along its        edges resulted only in 86% of all cases in S_(Rem.)≦51%.

After a running time of 10 days, samples were taken as in Example 1. Theanalysis results are: TABLE 2 Below tray 30 Above tray 32 Acrylic acid99.13% by weight 99.39% by weight

Example 2

As in Example 1. Everything was identical, except that the elementarycell of trays 30 to 32 was a face-centered square (as in FIG. 3, excepta=d=45 mm).

After a running time of 10 days, samples were taken as in Example 1. Theanalysis results are: TABLE 3 Below tray 30 Above tray 32 Acrylic acid99.09% by weight 99.59% by weight

Example 3

As in Example 1. Everything was identical, except that the elementarycell of trays 30 to 32 was rectangular as in FIG. 1. The edge lengthswere 28 mm and 35 mm.

After a running time of 10 days, samples were taken as in Example 1.

The analysis results are: TABLE 4 Below tray 30 Above tray 32 Acrylicacid 99.15% by weight 99.52% by weight

The experiments carried out resulted in the following order ofeffectiveness:

Example 2>Example 1>Example 3>Comparative Example.

1. A process for rectificatively separating one or more of (meth)acrylicmonomer-containing fluids, comprising feeding one or more of said fluidsinto a rectification column which comprises at least one sieve traywithout a runoff segment, wherein both an arrangement of the centers ofthe passages present in the sieve tray and an arrangement of thepassages themselves are regular, and can be obtained from a basic numberof these centers and passages, by defining a rectangular elementary cellin the sieve tray, which contains this basic number, and has four edges,of which two, in each cases are parallel to one another and of equallength, and this elementary cell shifts regularly and repeatedly alongits edges, the length of the shifting vector in each cases being thelength of the edge of the elementary cell along which the shifting iseffected.
 2. The process as claimed in claim 1, wherein the passages arecircular.
 3. The process as claimed in claim 1, wherein therectification column comprises more than one sieve tray, and wherein theonly separating internals of the rectification column are sieve trayswithout a runoff segment, as defined in claim
 1. 4. The process asclaimed in claim 3, wherein the passages in all sieve trays arecircular.
 5. The process as claimed in claim 1, wherein the(meth)acrylic monomer contained in at least one fluid is acrylic acid.6. The process as claimed in claim 5, wherein the acrylic acid contentof at least one fluid is from 2 to 5% by weight, or from 10 to 35% byweight, or ≧95% by weight.
 7. The process as claimed in claim 1, whereinat least one (meth)acrylic monomer-containing fluid comprises diphenylas solvent.
 8. The process as claimed in claim 1, wherein the elementarycell is a face-centered square.
 9. The process as claimed in claim 1,wherein the elementary cell is a rectangle.
 10. The A process as claimedin claim 1, wherein the elementary cell is a rhombus.
 11. The process asclaimed in claim 1, wherein at least one fluid is a liquid.
 12. Theprocess as claimed in claim 1, wherein at least one fluid is a gasmixture.
 13. The process as claimed in claim 2, wherein the(meth)acrylic monomer contained in at least one fluid is acrylic acid.14. The process as claimed in claim 3, wherein the (meth)acrylic monomercontained in at least one fluid is acrylic acid.
 15. The process asclaimed in claim 4, wherein the (meth)acrylic monomer contained in atleast one fluid is acrylic acid.
 16. The process as claimed in claim 2,wherein at least one (meth)acrylic monomer-containing fluid comprisesdiphenyl as solvent.
 17. The process as claimed in claim 3, wherein atleast one (meth)acrylic monomer-containing fluid comprises diphenyl assolvent.
 18. The process as claimed in claim 4, wherein at least one(meth)acrylic monomer-containing fluid comprises diphenyl as solvent.19. The process as claimed in claim 5, wherein at least one(meth)acrylic monomer-containing fluid comprises diphenyl as solvent.20. The process as claimed in claim 6, wherein at least one(meth)acrylic monomer-containing fluid comprises diphenyl as solvent.